Make nBlockSequenceId atomic
Make nBlockSequenceId atomic

Summary: Also skip assignement when not required.

Test Plan:
  make check
  ./test/functional/test_runner.py

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1958

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